Kinda of a standard clause.. (It was in my contract with 2 previous employers - both major consulting firms). However, very difficult, if not impossible to enforce.
As a matter of fact I have direct experience of that... roughly 3 years ago' with 3 colleagues of mine we left our then employer (a major Consulting firm) and set up a shop in Scandinavia. Of course we took some clients with us as well (and I mean major clients, expecially for the Scandinavian business). The company tried to legally enforce the clause and we were all sent a letter reminding us of our contractual obligations; a 12 month non competing clause (whereby for 12 months after you leave you cannot work for any client you have worked for, over the previous 12 months - either directly employed or on behalf of a competitor unless authorized to do so). We sought legal advice ( I had to do it in the UK because they wanted to take me to court here in England) and we were told that very often these letters are not followed up with "real" legal action, and are only meant to scare people off, as companies do not like negative pubblicity.
What we did then was to ask our the largest client (a major corporation in Scandinavia) to write to our previous employer stating that the wanted to be free to hire whoever they wanted (in this case us) and unless they immediately stopped the bullying they would never ever do any more business them.
It worked ... never heard of the guys in Victoria again!!
SO... DONT' WORRY